The present invention relates to a motor driven antenna apparatus mounted on an automobile and, more particularly, to an improvement in a case for holding a drive control mechanism for controlling extension and retraction of an antenna.
The motor driven antenna apparatus used for automobiles usually includes a telescopic antenna, an antenna holding tube for insertably holding the antenna, and a drive control mechanism for controlling extension and retraction of the antenna, held in the antenna holding tube, by the power of a motor.
The drive control mechanism includes a motor control circuit, a motor driven motor which is rotated forward and backward by the motor control circuit, a rope transfer mechanism which is rotated by the motor driven motor, and a rope which is transferred in its longitudinal direction by the rope transfer mechanism to push the antenna out of the antenna holding tube and pull it thereinto. The rope is formed of, e.g., synthetic resin having moderate hardness and elasticity. The drive control mechanism, which is held in the case constituted of hard synthetic resin or the like, is attached to the lower end portion of the antenna holding tube.
The motor driven antenna apparatus having the above constitution is mounted on the body of an automobile in the following manner. A portion of the antenna holding tube, which is located near the upper open end thereof, is fixed into a mounting hole formed in the body wall of the automobile, and a lower portion of the antenna holding tube is fixed into the automobile. Thus, when the antenna is not used, it is pulled into the antenna holding tube and retracted. When the antenna is used, it is pushed out of the antenna holding tube and extended.
The motor driven antenna apparatus for use in automobiles is usually mounted on the right or left side of a rear trunk of an automobile. However, there have recently been increasing in cars having no trunks, such as an RV (recreational vehicle) and a wagon. In such a car, a motor driven antenna apparatus is often attached to the front fender of the car.
While the engine of the car is acting, the fender has a considerably high temperature inside due to heat generated from the engine. Thus, the motor driven antenna apparatus attached to the fender, especially the inside of the case housing the drive control mechanism and mounted in the fender is considerably increased in temperature. Under the flaming sun in midsummer, even though the engine of the automobile is stopped, the inside of the fender is greatly increased in temperature and so is that of the case.
If the high-temperature case is cooled suddenly by any cause, condensation will occur in the case.
In a commonly-known car, a drain is formed under the windshield and an outlet communicating with the drain is provided on the side of the fender. At the time of rainfall or car washing, water flows from the windshield to the drain and is then discharged through the outlet at a stretch. It is likely that the discharged water will partly splash the high-temperature case arranged near the outlet.
Since the discharged water cools the case suddenly, there occurs a difference in temperature between the inside and outside of the case and accordingly condensation will be generated inside the case.
The foregoing case has a sealed structure in which it is sealed with a waterproof seal member to prevent water from externally entering through a gap between the body and cover of the case.
If condensation occurs in the case and a water drop is attached to a metal member of the drive control mechanism, especially the motor control circuit on the substrate to which a +B power supply voltage is always applied, electrolytic corrosion will occur on, e.g., the wiring of the circuit and so will general corrosion, resulting in malfunction.
In the above-described prior art motor driven antenna apparatus for use in automobiles, there is a case where the drive control mechanism holding case of the sealed structure is heated to a considerably high temperature and cooled suddenly by rainwater, a splash of discharged water at the time of car washing, or the like. In this case, there occurs a wide difference in temperature between the inside and outside of the case, with the result that condensation is caused in the case and motor driven corrosion or general corrosion is caused, thereby causing a malfunction.